To turn on a discharge lamp, it is necessary in the first place to produce a breakdown between electrodes of the discharge lamp by applying a high voltage across the electrodes. For generating the breakdown between the electrodes, an impulse high voltage is used which is generated across the secondary winding of an ignition (abbreviated to IGN from now on) transformer when the charges stored in a capacitor are discharged through the primary winding of the IGN transformer.
The turns ratio between the primary and secondary windings of the IGN transformer can be reduced by increasing the voltage to be discharged from the capacitor to the primary winding of the IGN transformer. In other words, increasing the voltage applied to the primary winding of the IGN transformer makes it possible to reduce the number of turns of the secondary winding. This enables using a thick wire to reduce resistance, and miniaturization of the IGN transformer. For this reason, the discharge lamp ballast apparatus has a circuit for applying a high voltage to the IGN transformer.
A conventional discharge lamp ballast apparatus has a step-up transformer for boosting the voltage of an AC power supply, and generates a DC high voltage through a voltage-doubler rectifier circuit composed of a plurality of diodes on the secondary side of the step-up transformer. The discharge lamp is started by charging the capacitor with the DC high voltage and by using the charges stored in the capacitor (see Patent Document 1, for example).
Alternatively, there is another apparatus which causes the secondary winding to output a boosted AC voltage by interrupting the DC voltage output from a ballast circuit with a switching device such as a saidac and by applying it to the primary winding of a transformer; generates a high voltage by supplying the AC voltage to a voltage-doubler rectifier circuit; and turns on the discharge lamp by charging the capacitor with the high voltage and by supplying the IGN transformer with the charges discharged from the capacitor in the same manner as described above (see Patent Document 2, for example).
In addition, there is still another apparatus which has a DC boosting circuit including a transformer, a DC-to-AC converter circuit and a start pulse generator circuit, and which turns on the discharge lamp by causing a capacitor in the start pulse generator circuit to output a high pulse voltage in synchronization with the operation of the DC-to-AC converter circuit. On the secondary side of the transformer of the DC boosting circuit, a winding is provided for generating the high voltage for starting the discharge lamp, and the start pulse circuit stores the power fed from the winding in the capacitor. When the voltage across the capacitor reaches a predetermined value, a self-breakdown-switching device in the start pulse circuit leads to breakdown. The pulse voltage generated at the breakdown is superimposed on a pulse voltage output from a bridge circuit constituting a DC-to-AC converter circuit, and the voltage is applied to the discharge lamp to turn it on (see Patent Document 3, for example).
The self-breakdown-switching device in the start pulse generator circuit has an air gap that leads to breakdown and is brought into conduction when a voltage about 1000 V is applied. In other words, the start pulse generator circuit charges the capacitor with the voltage higher than 1000 V. To generate the high voltage about 1000 V using the voltage-doubler rectifier circuit, it is necessary to input a high voltage greater than 500 V, or to construct the voltage-doubler rectifier circuit as a three or more voltage multiplier circuit if the input is about 300 V. To generate a high voltage about 1000 V, the circuit is configured using high-breakdown voltage components, or the circuit for generating the multiplied voltage with many components. In view of this, the foregoing apparatus obtains the high voltage by using the DC boosting circuit, DC-to-AC converter circuit and start pulse generator circuit. More practically, it is preferable to obtain the high voltage by generating a triple voltage.
Patent Document 1: Japanese patent application laid-open No. 59-196594/1984 (pages 2 and 3, and FIG. 2)
Patent Document 2: Japanese patent application laid-open No. 9-69393/1997 (pages 4 and 5, and FIG. 1)
Patent Document 3: Japanese patent application laid-open No. 7-142182/1995 (pages 3 and 4, and FIG. 1)
With such a configuration, the conventional discharge lamp ballast apparatus has the voltage-multiplier rectifier circuit placed on the output side of the DC-to-AC converter circuit (DC/AC inverter) that outputs a comparatively low frequency AC voltage. Thus, it not only takes a long time for recharging the capacitor of the IGN circuit for starting the discharge lamp, but also requires a capacitor with a large capacitance for the voltage-multiplier rectifier circuit. In addition, a large number of parts required to construct the circuit present a problem of making the miniaturization difficult.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a discharge lamp ballast apparatus capable of miniaturization and cost reduction by eliminating the winding for starting the discharge lamp from the transformer in the DC/DC converter, and by simplifying the voltage-multiplier rectifier circuit.